The invention relates to an apparatus for driving a record carrier, which apparatus comprises a transducer for generating a read signal in response to patterns recorded at the record carrier and displacement means for displacing the transducer and the record carrier with respect to each other,
which apparatus comprises auxiliary signal generating means for deriving an auxiliary signal from the read signal, which auxiliary signal has a relatively low frequency content as compared to the read signal,
binary signal generating means for generating a binary signal out of the auxiliary signal including first hold means for generating a first hold signal, second hold means for generating a second hold signal, signal combination means for generating a slicing signal out of the first hold signal and the second hold signal, and comparing means for comparing the auxiliary signal with the slicing signal and outputting a binary output signal,
which first hold means are adapted for temporarily maintaining the first hold signal at a value substantially corresponding to a top value of the auxiliary signal, which second hold means are adapted for temporarily maintaining the second hold signal substantially at a value corresponding to a bottom value of the auxiliary signal.
An apparatus as disclosed in the opening paragraph is known from European Patent Application EP 814 462. The read signal comprises relatively high-frequency components which represent information stored at the record carrier and comprises relatively low-frequency components which are used for control of the displacement means. The amplitude of the read signal may vary due to several causes, such as fingerprints on the disk. As a result the auxiliary signal varies around a center value which itself is variable. In the known apparatus the signal combination means, formed by an adder and a multiplier, receive a first hold signal from a top hold detector and a second hold signal from a bottom hold detector. The value of the first hold signal gradually diminishes during intervals where said output value is higher than the auxiliary signal at the input of the top hold detector. The combination of top hold detector, bottom hold detector and output means enables the apparatus to adapt the slicing level to relatively slow changes in the center value, so that despite such slow changes a reliable binary output signal is obtained. In addition the apparatus has a defect signal detecting unit for holding at least one of the top hold detector and the bottom hold detector if no reliable auxiliary signal is available. It is a disadvantage of the known apparatus that the binary signal generating means cannot follow fast changes in the center value of the auxiliary signal.
It is a purpose of the invention to provide an apparatus according to the introductory paragraph in which this disadvantage is avoided. For this purpose the apparatus according to the invention is characterized in that the first hold means are further adapted for temporarily maintaining the value of the first hold signal at a value substantially corresponding to a bottom value of the auxiliary signal augmented by an addition value and in that the second hold means are further adapted for temporarily maintaining the value of the second hold signal at a value corresponding to a top value of the auxiliary signal diminished by a subtraction value.
In the circumstances that the auxiliary signal varies around a constant center value or a slowly variating center level, the value of the first hold signal remains less than the auxiliary signal augmented by an addition value. Likewise the value of the second hold signal remains higher than the value of the auxiliary signal diminished by a subtraction value. Hence in those circumstances the slicing level is based on the top hold value and the bottom hold value of the auxiliary signal. If however the value of the auxiliary signal augmented with the addition value (augmented value) decreases until a value lower than the present top value, then the output of the first hold means is determined by the augmented value. Likewise if the value of the auxiliary signal diminished with the subtraction value (diminished value) increases until a value higher than the present bottom value then the output of the second hold means is determined by the diminished value. This allows the binary signal generation means to rapidly adapt to variations in properties of the auxiliary signal.
It is remarked that U.S. Pat. No. 5,412,692 discloses a data which has binary signal generating means comprising a maximum value detector, a minimum value detector, a voltage shift-down circuit and a voltage shift up circuit. The slicing level is determined by the average value of the signals generated by the voltage shift-down circuit and the voltage shift-up circuit. This average value generally has an error which is proportional to the difference between the values generated by the shift-up circuit and that by the shift-down circuit. The influence of said error on the accuracy of the binary output signal increases however during intervals wherein the first auxiliary signal has a small amplitude. In the apparatus of the invention the slicing signal is computed from the top hold value and the bottom hold value in those circumstances. Hence the difference between the top value and the bottom value, and therewith the error in the slicing signal is also small.